The present invention relates to a photo enhanced CVD apparatus for depositing thin film product on the basis of in opto-chemical vapor phase reaction.
Recently, in the field of thin film formation process for non-single-crystalline silicon semiconductors or for passivation of semiconductor devices, chemical vapor deposition enhanced by optical energy (hereafter called Photo CVD) has aroused interest to artisans.
The Photo CVD process comprises disposing a substrate in a reaction chamber, inputting process gas into the reaction chamber at a negative pressure, initiating opto-chemical vapor reaction by irradiating the process gas with ultraviolet light and depositing the product of the reaction on the substrate in the form of a thin film. The ultraviolet light is radiated from an ultraviolet soruce through a transmissive window which is made of quartz glass, including lithium fluoride or magnesium fluoride or so on. Together with process gas, mercury may be introduced to accelerate the deposition speed. The window for ultraviolet light can be designed with a larger dimension by reducing the pressure in the light source room.
There are some problems in such a Photo CVD process. Because process gas is in contact with the window for ultraviolet light, the opto-chemical reaction takes place also near the window by ultraviolet light passing therethrough, and therefore the product is deposited on the window during the process and deteriorates the transparency of the window. For this reason, the window eventually become opaque and the opto-chemical reaction is quenched.
To solve the above problem, it has been proposed to coat the window with oil to prevent the formation of the product on the window. The coated oil, however, substantially degrades the quality of film deposited by comtaminating the film with its ingredient.
Another problem is that the temperature of the light source substantially rises as it is emitting ultraviolet light, due to radiation heat from a heater for substrate heating or due to heat induced by the light source itself. The temperature rises, e.g., 300.degree. C. At such a high temperature, the intensity distribution of ultraviolet light is changed, particularly in the wavelength range less than 300 nm, it is reduced by 50%, and therefore the deposition speed becomes lowered very much.
Further, it may be also a problem that the cost of system for this process becomes high because of the high cost of quartz glass and magnesium fluoride.
Still further, such a Photo CVD process is not suitable for mass-production because of the very low deposition speed.